By radioligand assay, receptors for protein and peptide hormones are generally ascribed to cell membrane fractions. By immunocytochemistry, most of these receptors are found intracellularly. We would like to explore the physiologic significance of these intracellular receptors using as a model, receptors for luteinizing hormone-releasing hormone (LHRH) on secretory granules of pituitary gonadotrophs. To investigate the relationship of these receptors with other functions of the cell, receptors reached with LHRH, cyclic AMP, cyclic GMP and pineal factor in vivo (occupied receptors) as well as receptors reactive with these four ligands in vitro (free receptors) will be assayed in rats at varying time intervals after intravenous injection of LHRH and pineal peptide. The subcellular distribution of injected hormone and cell-generated cyclic nucleotides will be correlated with morphological observation of degranulation and castration cell formation. The time sequence of appearance and disappearance of cell membrane and intracellular hormone binding will be interpreted in relation to release of granule contents, recycling and regeneration of hormone receptors, generation and distribution of cyclic nucleotides and a possuble dual external control of cellular physiology, as a general biologic phenomenon.